1. Field of the Invention
The present invention relates to a lithium battery comprising a positive electrode, a negative electrode, and a nonaqueous electrolyte containing a solute and a solvent, and more particularly, to a lithium battery improved in charge/discharge cycle performance through suppression of reaction between a positive-electrode active material of the positive electrode and the nonaqueous electrolyte.
2. Description of the Related Art
Recently, rechargeable batteries have found applications in various fields such as electronics. As a novel battery of high power and high energy density, in particular, lithium batteries featuring high electromotive force derived from oxidation/reduction of lithium in the nonaqueous electrolyte have come into wide use.
Such lithium batteries have conventionally employed various metal oxides capable of absorbing and desorbing lithium ions as the positive-electrode active material for use in the positive electrode. More recently, studies have been made on the use of manganese oxides, such as manganese dioxide, as the positive-electrode active material of the lithium battery because manganese oxides generally provide high discharge potentials and are inexpensive.
Unfortunately, in charge/discharge processes of the lithium battery including the positive-electrode active material of manganese oxide, the manganese oxide is repeatedly expanded and contracted so that the crystal structure thereof is destroyed. As a result, the battery suffers a degraded charge/discharge cycle performance.
In recent attempts to improve the charge/discharge cycle performance of the lithium battery including the positive-electrode active material of manganese oxide, a variety of positive-electrode active materials have been proposed. For instance, Japanese Unexamined Patent Publication No. 63-114064(1988) discloses a positive-electrode active material comprising a lithium-manganese complex oxide obtained from manganese dioxide and Li2MnO3. Japanese Unexamined Patent Publication No. 1-235158 (1989) provides a positive-electrode active material comprising a complex oxide of lithium-containing manganese dioxide wherein lithium is incorporated in the crystal lattice of manganese dioxide. Further, Japanese Unexamined Patent Publication Nos. 4-237970(1992) and 9-265984(1997) disclose positive-electrode active materials comprising lithium-manganese complex oxides added with boron.
The invention is directed to a lithium battery comprising a positive electrode, a negative electrode, and a nonaqueous electrolyte containing a solute and a solvent, the battery adapted to suppress the reaction between a positive-electrode active material of the positive electrode and the nonaqueous electrolyte for achieving an excellent charge/discharge cycle performance.
A lithium battery according to the invention comprises a positive electrode, a negative electrode, and a nonaqueous electrolyte containing a solute and a solvent, wherein the positive electrode comprises a positive-electrode active material of boron-containing lithium-manganese complex oxide prepared using manganese dioxide with a specific surface area of 15 to 50 m2/g.
According to the inventive lithium battery wherein the positive electrode comprises the positive-electrode active material of boron-containing lithium-manganese complex oxide prepared using manganese dioxide with the specific surface area of 15 to 50 m2/g, boron in the positive-electrode active material suppresses the reaction between the lithium-manganese complex oxide and the nonaqueous electrolyte during charging. Besides, the boron-containing lithium-manganese complex oxide has the specific surface area in such a suitable range as to obviate a problem that the boron-containing lithium-manganese complex oxide has too great a specific surface area or too great a contact area with the nonaqueous electrolyte, tending to react with the nonaqueous electrolyte. Thus, the reaction between the positive-electrode active material and the nonaqueous electrolyte is more positively suppressed. As a result, the positive-electrode active material is prevented from being dissolved in the nonaqueous electrolyte, so that increase in the internal pressure of the lithium battery is suppressed. Hence, the battery is improved in the charge/discharge cycle performance.
If the boron-containing lithium-manganese complex oxide as the positive-electrode active material has a specific surface area of less than 12 m2/g, the current density during the charge/discharge process increases thereby to increase polarization of the positive electrode. This results in an increased possibility of side reaction wherein the nonaqueous electrolyte is decomposed. If the boron-containing lithium-manganese complex oxide as the positive-electrode active material has a specific surface area in excess of 45 m2/g, the positive-electrode active material is increased in contact area with the nonaqueous electrolyte, thus becoming more prone to react with the nonaqueous electrolyte. Therefore, the positive-electrode active material of boron-containing lithium-manganese complex oxide may preferably have a specific surface area in the range of 12 to 45 m2/g. Such a boron-containing lithium-manganese complex oxide more positively suppresses the increase in the internal pressure of the lithium battery, thereby even further improving the charge/discharge cycle performance of the lithium battery.
For more positive suppression of the reaction between the positive-electrode active material and the nonaqueous electrolyte during charging, the solute in the nonaqueous electrolyte of the inventive battery may preferably include at least one substance selected from the group consisting of lithium trifluoromethanesulfonimide, lithium pentafluoroethanesulfonimide, lithium trifluoromethanesulfonmethide, lithium trifluoromethanesulfonate and lithium hexafluorophosphate. More preferably, the solute may include at least one substance selected from the group consisting of lithium trifluoromethanesulfonate and lithium trifluoromethanesulfonimide.
According to the inventive lithium battery, known solvents generally used in the art may be employed as the solvent for the nonaqueous electrolyte. However, for particular purposes of suppressing the reaction between the nonaqueous electrolyte and the positive-electrode active material as well as of increasing the ionic conductivity of the nonaqueous electrolyte, it is preferred to use a solvent mixture containing at least one organic solvent selected from the group consisting of ethylene carbonate, propylene carbonate, butylene carbonate, vinylene carbonate, xcex3-butyrolactone and sulfolane, and at least one organic solvent selected from the group consisting of 1,2-dimethoxyethane, 1,2-diethoxyethane, 1,2-ethoxymethoxyethane, tetrahydrofuran, dioxolane, dimethyl carbonate, diethyl carbonate and ethylmethyl carbonate. More preferred is a solvent mixture containing at least one organic solvent selected from the group of propylene carbonate and ethylene carbonate, and 1,2-dimethoxyethane.
For proper suppression of the reaction between the positive-electrode active material and the nonaqueous electrolyte during charging, the solvent mixture may preferably contain the two types of organic solvents in respective concentrations of not less than 10 vol%.
According to the inventive lithium battery, the boron-containing lithium-manganese complex oxide as the positive-electrode active material may be obtained by heat-treating a mixture of a boron compound, a lithium compound and manganese dioxide in the presence of oxygen, the mixture containing boron, lithium and manganese in an atomic ratio (B:Li:Mn) of 0.01-0.20:0.2-2.0:1.
Such a composition offers lithium-manganese complex oxide crystals incorporating therein boron or boron compound in a boron-to-manganese atomic ratio (B/Mn) in the range of 0.01 to 0.20.
If the lithium-manganese complex oxide crystals incorporate therein boron or boron compound in an atomic ratio (B/Mn) of 0.01 to 0.20, boron in the positive-electrode active material contributes an adequate suppression of the reaction between the lithium-manganese complex oxide and the nonaqueous electrolyte during charging. Besides, such a positive-electrode active material obviates a problem that boron uninvolved in the charge/discharge process accounts for too great a portion to form a proper solid solution with the lithium-manganese complex oxide whereby the positive-electrode active material suffers an instable crystal structure. As a result, the lithium battery is improved in the charge/discharge cycle performance with the suppressed reaction of the positive-electrode active material with the nonaqueous electrolyte.
In the preparation of the boron-containing lithium-manganese complex oxide, examples of a usable boron compound include boron oxide B203, boric acid H3BO3, metaboric acid HBO2, lithium metaborate LiBO2, quaternary lithium borate Li2B4O7 and the like. Examples of a usable lithium compound include lithium hydroxide LiOH, lithium carbonate Li2CO31 lithium oxide Li2O, lithium nitrate LiNO3 and the like. Examples of a usable manganese compound include manganese dioxide MnO2, manganese oxyhydroxide MnOOH and the like.
In the heat-treatment of the boron compound, lithium compound and manganese dioxide for producing the positive-electrode active material, temperatures below 150xc2x0 C. will result in insufficient incorporation of boron or boron compound into the lithium-manganese complex oxide solid and also in insufficient removal of water of crystallization of manganese dioxide. The residual water of crystallization reacts with lithium so as to degrade storability of the lithium battery. On the other hand, heat-treatment temperatures in excess of 430xc2x0 C. will result in decomposed manganese dioxide so that the resultant complex oxide presents an insufficient mean manganese valence. This leads to the instable crystal structure of the positive-electrode active material, which, in turn, tends to react with the nonaqueous electrolyte, degrading the charge/discharge cycle performance of the lithium battery. Therefore, the boron compound, lithium compound and manganese compound may be heat-treated at temperatures of 150xc2x0 C. to 430xc2x0 C., preferably of 250xc2x0 C. to 430xc2x0 C., or more preferably of 300xc2x0 C. to 430xc2x0 C.
If the boron compound, lithium compound and manganese compound are heat-treated in such a manner, boron or the boron compound form a proper solid solution with the lithium-manganese complex oxide without altering the crystal structure thereof. Thus is maintained the crystal structure combining Li2MnO3 and MnO2 and featuring an excellent charge/discharge cycle performance.
In the inventive lithium battery, examples of a usable negative-electrode active material of the negative electrode include lithium metals generally used in the art; lithium alloys such as lithium-aluminum alloy, lithium-lead alloy, lithium-tin alloy and the like; and carbon materials capable of absorbing and desorbing lithium ions such as graphite, coke and the like. Where the negative-electrode active material is a lithium-aluminum alloy, in particular, the nonaqueous electrolyte forms an ion conductive film over a surface of the negative-electrode active material. The film serves to suppress the reaction of the negative-electrode active material with the nonaqueous electrolyte, thereby further improving the charge/discharge cycle performance of the lithium battery.
These and other objects, advantages and features of the invention will become apparent from the following description thereof taken in conjunction with the accompanying drawings which illustrate specific embodiments of the invention.